Betaines of the above-named formula have gained increasing importance in past years for the preparation of body cleansing and personal care agents. They combine excellent cleansing properties with good skin tolerance. In aqueous solution, the betaines form a stable, dense foam, which does not collapse even in the presence of soap.
The synthesis of such betaines is described in many patents, of which U.S. Pat. No. 3,225,074 is cited as representative. In general, the corresponding tertiary fatty acid amidamine is reacted with the alkali salt of a halogencarboxylic acid, usually chloroacetic acid. The reaction takes place in an aqueous medium. The alkali chloride, formed by the reaction, is left in this aqueous betaine solution. Such betaines are therefore generally marketed at a concentration of about 30 weight percent in the form of aqueous or alkaline aqueous solutions.
There has been no lack of attempts to prepare more concentrated solutions of the betaines, in order to reduce the costs of the transport and storage of the betaine solutions.
The expert knows that the viscosity of an aqueous surfactant solution also increases as the concentration increases. Frequently, however, it turns out that, when the concentration exceeds a value of about 60 to 70 weight percent, the viscosity decreases to a minimum with further increases in concentration and then rises again steeply. To explain this viscosity anomaly, it is assumed that a phase (the so-called G phase) with a lamellar structure is formed in the solution. This decrease in the viscosity of surfactant solutions at a concentration of about 60 to 70 weight percent is utilized for the preparation of highly concentrated surfactant solutions and is described, for example, in "Soap, Perfumery, Cosmetics", 1982, 507 to 509.
This viscosity anomaly also forms the background for the method of the German Patent No. 2,557,456, in which, for the synthesis of amine oxides by the oxidation of amines in the presence of water, such an amount of water is added, that the water concentration in the product mixture is 20 to 30 weight percent. In this method, efforts thus are already made during the synthesis to attain a surfactant solution, which corresponds to the viscosity minimum at high concentrations.
If water is removed from betaine solutions obtained in the usual manner, the viscosity of these solutions increases. However, at a concentration of about 40 weight percent betaine, the solution become pasty and increasingly solid as further water is removed.
Liquefaction on drying the solid product further has not been observed. Therefore, according to the state of the art, it was not possible to obtain highly concentrated solutions, which contain at least 70 weight percent of betaine of the general formula I and which are still flowable and can be metered, for example, with pumps.